Spectrum analyzers can be used for determining a noise figure of components like amplifiers or mixers. A known method for determining the noise figure is the so called Y-method, which is for example shown in the document US 2005/0137814 A1. This method comprises connecting a diode, such as an Enhanced Noise Ratio diode (ENR-diode) to the device under test (DUT) and successively switching between a regular noise signal and an enhanced noise signal. The spectrum analyzer then measures the noise power level in both situations and can determine the noise figure and the amplification factor of the DUT therefrom. The accuracy of the measuring system though is strongly influenced by a noise figure of the employed measuring device (e.g., the employed spectrum analyzer). For reducing the noise figure of the measuring device, it is suggested to use a low noise pre-amplifier (LNA). It is thereby possible to significantly reduce the noise figure of the measuring system. This, however, also leads to a reduction of the available dynamic range. Especially in broadband applications, it is possible to overpower the first stage of the analyzer with the power of the pre-amplified measuring signal.
What is needed, therefore, is a measuring system and measuring method that allow for a very accurate measurement of the noise figure and amplification of a device under test, independent of the power of the measuring signal.